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From: "Pierre Muller" <muller AT ics DOT u-strasbg DOT fr>
To: <djgpp-workers AT delorie DOT com>
Subject: 3rd try: SSE2 sigsegv due to alignment problem
Date: Tue, 11 Sep 2007 09:25:38 +0200
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Reply-To: djgpp-workers AT delorie DOT com

I sent this message in April, but it never reached the mailing list :(
I finally asked DJ to update my email address
in the mailing list.

Thanks DJ

>> Original message starts here

Hi,

  I am testing the next release of Free Pascal for go32v2.
This release uses an internal copy of the linker script that seems to
generate a problem regarding the
16 byte alignment requirement of memory reading or writing xmm registers.

the following code is the source of the problem:

uses
   mmx;

{ only a small test to see if it works in principal }

var
  b : byte;
  q : array[0..15] of byte;

begin
  if is_sse2_cpu then
    asm
       movdqa    %xmm1,%xmm2
       movdqa    q,%xmm4
       psubq     %xmm1,%xmm2
       psubq     q,%xmm4
   end;
end;


the allocation of B and Q variables are made using .lcomm but when I compile
with stabs info, I discover that both B and Q are at an address ending with
0x8.
Looking into the linker script, I discovered that the .bss section has some
code:
  ScriptRes.Add('    .bss  SIZEOF(.data) + ADDR(.data) :');
  ScriptRes.Add('    {');
  ScriptRes.Add('      _object.2 = . ;');
  ScriptRes.Add('      . += 24 ;');
  ScriptRes.Add('      *(.bss)');

which reflects the
  .bss  SIZEOF(.data) + ADDR(.data) :
  {                                   
    _object.2 = . ;
    . += 24 ;
    *(.bss .bss.* .gnu.linkonce.b.*)

found in CVS revision 1.11 of
djgpp/lib/djgpp.djl
which is the default linker script if I am not mistaken.

I have no clue of the use of this 24 reserved bytes, but if I change this to

   . += 32 ;
then both B and Q variables are aligned at a 16 byte boundary, and the code
does not crash anymore.

Question 1: what is the use of these 24 bytes?
Question 2: is there any problems with increasing this to 32 bytes, in order
to get a correct alignment?
(I noticed that the first space allocated in sel_buf, so if there is code
that assumes that sel_buf is at
24 bytes from object.2 symbol, this might lead to
problems.)

Pierre Muller

PS: sorry, but I was unable to translate the code above into djgpp assembler
inside C.